The immune response to most proteinaceous antigens requires the productive interaction of B lymphocytes, helper T (Th) lymphocytes, and macrophages. Communication among these cells is mediated by both cell membrane-associated ligands and soluble interleukins. The primary ligand-receptor interaction which occurs during cognate Th cell-B cell interactions involves Th cell antigen receptor (alpha/betaTCR) binding to peptide-associated MHC class II molecules (Ia) and leads to Th cell activation. Recent studies suggest that signaling which occurs during this interaction may be bidirectional, leading to transduction of essential growth promoting signals through B cell Ia molecules. Specifically, anti-Ia antibody mediated cross-linking of less than 5 percent of Ia on IL-4 and antigen surrogate (anti-mu) primed B lymphocytes inducescell activation, proliferation and differentiation. Interestingly, during the priming period, Ia coupling to second messenger generating systems changes so that crosslinking of Ia now triggers tyrosine kinase dependent activation of a PtdInsP2 phospholipase C, and mobilization of Ca++. Crosslinking of Ia on these cells also leads to rapid phosphorylation of a 55 kD Ia-associated 2Kprotein (pp55). Based on these findings, the applicant hypothesizes that Ia molecules expressed on antigen and IL-4 primed B cells transduce growth promoting signals during cognate Th cell-B cell interactions. Then these signals are mediated, at least in part, via stimulation of a tyrosine kinase(s), potentially pp55, which activates inositol lipid hydrolysis, perhaps by phosphorylation of PLCgamma2, leading to Ca++ mobilization. This application proposes to address this hypothesis by defining the structural basis and the physiologic importance of Ia-mediated signal transduction leading to tyrosine kinase activation, PtdInsP2 hydrolysis, and Ca++ mobilization. In Aim 1, the applicant proposes to employ T cells and genetically engineered CD4 and alpha/betaTCR fusion proteins to determine whether Ia ligation by the physiologic Ia binding ligands alpha/betaTCR and CD4 activates signal transduction. Aim 2 proposes the characterization of pp55 including physicochemical analysis, cloning and sequencing, and (with Aim 3) will address the possibility that pp55 is an Ia-regulated src family kinase. Studies in Aim 3 will quantitate tyrosine kinase activation following Ia crosslinking and determine whether PLCgamma2 is tyrosine phosphorylated and thereby activated during Ia- mediated signaling. Studies proposed in Aim 4 will utilize transfectants made using genetic constructs which encode mutant I-Aalpha and beta chains to determine Ia structural requirements for association with pp55, and coupling to tyrosine kinase activation and Ca++ mobilization. Finally, studies proposed in Aim 5 will utilize transgenic mice which express these mutant (non-signaling) Ia molecules to determine the role of specific Ia- mediated signal transduction events in generation of Th-cell dependent immune responses. The studies should provide important insight regarding the basis of T cell-mediated B signaling which is critical for immune defense against many potential pathogens.